Optical communication systems are configured to transmit signal light through optical fiber as a transmission medium, thereby enabling fast transmission/reception of large volume of information. Wavelength Division Multiplexing (WDM) optical communication systems are configured to transmit signal light in which multiple channels of different wavelengths are multiplexed (multiplexed signal light), thereby enabling transmission/reception of larger volume of information. In recent years, there are demands for further increase of volume in the WDM optical communication systems and under such circumstances there are ideas to narrow the signal channel spacing in WDM optical transmission and to expand the wavelength band used in WDM optical transmission.
In order to expand the signal wavelength band, studies have been conducted to utilize the initial C-band (1530 nm-1565 nm) and also to use the L-band (1565 nm-1625 nm) and the U-band (1625 nm-1675 nm) on the longer wavelength side than the C-band. In addition, there are also studies to utilize the O-band (1260 nm-1360 nm), the E-band (1360 nm-1460 nm), and the S-band (1460 nm-1530 nm) on the shorter wavelength side than the C-band.
The optical fibers to transmit signal light in such a broad band are demanded to satisfy conditions such as a small transmission loss in the signal wavelength band. The optical fibers applied to the optical communication systems are generally transmission media consisting primarily of silica glass, which have a minimum transmission loss near the wavelength of 1550 nm in the C-band and an increase of loss due to OH-radical at the wavelength of 1380 nm.
Characteristics of standard single-mode optical fiber are defined by International Standard (ITU-T G.652). According to this Standard, a standard single-mode optical fiber has the zero dispersion wavelength of 1300 nm-1324 nm, the mode field diameter whose center value at the wavelength of 1310 nm is 8.6 μm-9.5 μm and whose tolerances of deviation are ±0.7 μm, and the cable cutoff wavelength of 1260 nm or less. Single-mode optical fibers compliant with this Standard are manufactured and sold by many optical fiber makers, for example, as described in Non-patent Documents 1 to 4.
On the other hand, as an optical fiber for the 1.55 μm wavelength band, for example, Non-patent Document 5 discloses an optical fiber having the transmission loss of 0.154 dB/km at the wavelength of 1550 nm, the transmission loss of 0.291 dB/km at the wavelength of 1300 nm, and the OH-related loss increase of 0.75 dB/km at the wavelength of 1380 nm. Additionally, Patent Document 1 discloses an optical fiber having the transmission loss of 0.170-0.173 dB/km at the wavelength of 1550 nm and the OH-related loss increase of 0.3 dB/km at the wavelength of 1380 nm.
Patent Document 1: U.S. Pat. No. 6,449,415
Non-patent Document 1: Catalog of Sumitomo Electric Industries, Ltd., “Specification for Low Water Peak Single-Mode Optical Fiber (G.652D) <PureBand™>,” Aug. 25, 2003
Non-patent Document 2: Catalog of Corning Inc., “Corning R SMF-28e™ Optical Fiber Product Information,” March 2003
Non-patent Document 3: Catalog of OFS, “AllWave R Fiber The New Standard for Single-Mode Fiber,” 2003
Non-patent Document 4: Catalog of Alcatel, “Alcatel 6901 Enhanced Singlemode Fiber,” January 2002
Non-patent Document 5: Hiroshi Yokota et al., “Loss characteristics of ultralow-loss pure silica core single-mode fiber,” THE INSTITUTE OF ELECTRONICS AND COMMUNICATION ENGINEERS, 1986 General Conference, 1091